1. Field of the Invention
The present invention relates to methods and apparatus for mounting integrated circuits on a printed circuit (PC) board and more particularly to such methods and apparatus which achieve electrical connection between the integrated circuit and the PC board without soldering.
2. Description of the Related Art
Tape assisted bonding (TAB) is a technique for connecting conductive leads to the electrical contacts on an integrated circuit chip, known as a die. When such leads are so connected, they can be used to interconnect the integrated circuit with additional circuitry. Typically a strip of film is divided into a plurality of adjacent rectangular segments, each of which has a plurality of conductive leads etched thereon. A die mounted on a central portion of each segment is electrically connected to the conductive leads in a known manner. When so assembled, the film or tape upon which the die are mounted may be wound onto reels prior to additional processing, testing and the like. After final testing, the tape is cut into individual TAB segments, each of which contains a die and an associated set of conductive leads.
Sometimes, such TAB segments are mounted on PC boards. The PC boards include a plurality of conductive leads, sometimes referred to as traces or conductive metallization, with which corresponding TAB segment leads are first aligned and then electrically and mechanically connected via soldering. Mounting TAB segments on PC boards with soldered connections is disadvantageous for several reasons.
First, because of the possibility of solder bridging between adjacent leads, the leads on the TAB segment must be precisely aligned opposite the corresponding PC board leads to prevent solder bridging between a TAB lead and an adjacent PC board lead. Such alignment requires expensive equipment which utilizes video techniques as opposed to much cheaper alignment systems which utilize optical reflectance methods. In addition to the capital cost of the alignment system, there is additional expense associated with specialized soldering equipment adapted to solder TAB segment leads to PC board leads and the cleaning equipment which is utilized after soldering. Waste disposal of substances such as fluorocarbons, fluxes and lead also presents additional operational expenses when soldering is used.
The heat associated with the soldering process is also disadvantageous in several respects. It is undesirable to expose the integrated circuits on the TAB segments to the heat necessary for melting the solder to make the connection. Some circuits are more sensitive than others and circuit reliability may be affected by the heat. Because TAB soldering techniques are often different from those required to solder other components to a PC board, either the TAB segments are soldered first and thereafter the other components, or vice versa. In either case, the solder connections which are first made may be weakened or become completely disconnected when heat is applied to make the second solder connections.
The field replacement of TAB segments which are soldered to PC boards is difficult, and in some cases impossible, because of the close lead spacing and the alignment tolerances. Such replacement for systems in the field is desirable to make repairs, to enhance performance or to add a capability to the system in which the PC board is installed.
Finally, the assembly cycle time required for soldering a typical TAB segment to a PC board is about 5 minutes. It would be advantageous to reduce this time considerably.